1. Field of the Invention
This invention relates generally to the manufacture of semiconductor devices. More particularly, the invention pertains to methods and apparatus for clamping portions of leadframes for making conductive wire connections (wire bonds) between portions of the leadframe and the bond pads of the semiconductor device.
2. State of the Art
Semiconductor packages are formed in a variety of different designs. Among the various package configurations are dual in-line packages (DIP), zig-zag inline packages (ZIP), small outline J-bends (SOJ), multi-level leads-on-chip (MLLOC), tape-under-frame (TUF), thin small-outline packages (TSOP), plastic leaded chip carriers (PLCC), small outline integrated circuit (SOIC), plastic quad flat pack (PQFP), thin quad flat pack (TQFP), and interdigitated leadframe (IDF). Wirebonding of each type of package requires a leadframe clamping device with particular dimensions and/or features.
In general, all package designs have several common elements. These include a sealed package enclosure, a die-attachment area, bonding wires for making electrical contact between the bond pads of the die and the leads of the package, and the inner and outer portions of leadfingers of the metal lead system of the package.
Typically, the leads of a leadframe for a semiconductor device are first formed in multiple pattern units in a leadframe, a metal strip with multiple bonding sites, each of which provides the leads for the packaged device and, in some instances, may provide support for the semiconductor device. A typical conventional leadframe strip is produced from metal sheet stock, such as a copper alloy, and has xe2x80x9cpaddlesxe2x80x9d upon which the semiconductor devices are mounted. During the wirebonding process the leadframe strip is moved and indexed from bonding site to bonding site through a clamping apparatus which retains the leadframe strip at sequential bonding sites for producing a plurality of wire-bonded semiconductor devices. The typical conventional bonding machine is designed with parallel non-resilient upper and lower clamping surfaces.
The wirebonding process comprises attaching fine or small diameter wires to bond pads on the semiconductor device and to portions of the leadfingers of the leadframe strip. The wirebonded semiconductor devices are then further processed or encapsulated into packaged semiconductor devices.
As manufactured, leadframe strips typically vary in width, camber, and thickness. For example, a leadframe strip may vary in thickness from edge to edge. Even small differences in thickness from one leadfinger to another may significantly affect the clamping effectiveness during the wire bonding process. Leadfingers which move during the wire bonding process may tend to be insufficiently bonded or have poor wire bonds, and, as a result, ultimately fail.
The result of such leadframe variations has been recognized for a long time, and the patent literature shows various apparatus directed toward resolving the problem.
U.S. Pat. No. 4,765,531 of Ricketson et al. discloses a wirebonding workstation with a planar upper clamp plate having a window.
U.S. Pat. No. 5,307,978 of Ricketson et al. discloses a leadframe clamping apparatus having an upper clamp plate with a window. This is a conventional clamp plate used on Kulicke and Soffa wirebonding machines.
U.S. Pat. No. 5,647,528 of Ball et al. and 5,197,652 of Yamazaki disclose dual leadframe clamping apparatus. Primary fixed clamps are augmented with a secondary independent clamp which moves with the bonding apparatus from leadfinger to leadfinger. The independent clamp of Ball et al. may include insulation or cushioning on its end, or be equipped with a spring, to control the compression force on the individual leadfinger.
U.S. Pat. No. 3,566,207 of Adams discloses a clamping apparatus for holding a leadframe in place while an integrated circuit chip is bonded to a chip mounting pad. The clamping apparatus consists of a pair of narrow clamps, each of which is pressed downward on a leadfinger of the leadframe.
In U.S. Pat. No. 3,685,137 of Gardiner, multiple anvils are positioned over leadfingers to clamp them downwardly against a plunger.
U.S. Pat. No. 4,821,945 of Chase et al. describes a leadframe clamping apparatus, in which a clamp adjacent the wirebonding capillary clamps the leadfinger being wirebonded against movement.
For example, U.S. Pat. No. 5,035,034 of Cotney discloses a metal clamp frame with a clamp insert. The insert includes a series of flexible fingers, each of which is positioned to clamp one of the leadfingers on the leadframe. The Cotney invention requires a separate clamp insert for every device having a slightly different pattern, number or length of leadfingers. Thus, each insert is very product-specific. Moreover, the manufacture of such a clamp insert wherein all of the flexible fingers provide the same clamping force, even after extended use, is conceivably difficult and expensive. The individual clamping fingers are fragile and easy to distort. Weakening or breakage of a single flexible finger will require replacement and/or the fabrication of a completely new insert.
U.S. Pat. No. 5,322,207 of Fogal et al. describes a leadframe clamp with dual wirebonding windows for wirebonding two semiconductor dice at a time, i.e. without moving the leadframe. A heating block is configured to simultaneously heat the paddle contact areas of both leadframe xe2x80x9cframesxe2x80x9d.
While prior art clamping devices are acceptable, they suffer from a common drawback, i.e., the inability to easily accommodate dimensional variations in the leadframe strip. Wirebond failures resulting from such variations are unacceptable in the current state of integrated circuit (IC) semiconductor device manufacture. A self-leveling clamping apparatus is needed which is inexpensive to construct, quickly adaptable to all leadframe sizes and types, permits rapid wire-bonding of the leads of the leadframe to the bond pads of the semiconductor device, and which significantly reduces damage to wirebonds and leads.
The invention comprises an improvement in an apparatus for clamping a substrate; e.g., leadframe to a wirebonding machine for performing an integrated circuit wirebonding operation. In the invention, a polymeric clamp insert with a wirebonding window and a clamping surface is inserted in a clamp holder. A thin resilient membrane is placed above the clamp insert. The clamp insert has sufficient xe2x80x9cplayxe2x80x9d or latitude of movement so that the clamp insert is self-leveling or self-adjusting on the leadframe upon application of a clamping force. The resilient member ensures that the clamp insert is nearly uniformly compressed downwardly on the leadfingers about the die, irrespective of side-to-side or end-to-end thickness variations in the leadframe.
The resilient member also acts as a dampener to absorb mechanical shocks which may otherwise damage the leadframe, die or bondwires.
The apparatus of the invention may be incorporated in a multi-member self-leveling or self-adjusting insert member which is inserted into a clamp insert carrier. The insert member has a wirebonding window surrounded by a narrow clamping surface wherein compensation for non-uniform leadframe thickness is provided. Preferably, the insert member is formed of a polymer whereby additional advantages accrue from both the non-conductive electrical property, the low heat conductive property of the insert device, and the degree of elasticity in the polymeric leadframe insert itself. The insert member is configured to provide the required compensation without permanent deformation.
In one form the clamp apparatus of the invention includes (a) a clamp insert holder with an insert aperture therethrough, (b) a clamp insert with a bonding window and a clamping surface to contact the leadframe, (c) an elastic member formed of a polymeric material, e.g. a polytetrafluoroethylene material or a urethane material, and (d) a retaining member which retains the elastic member in a position biased against the insert.
The insert has a generally peripheral clamping surface or ridge, and is formed of a non-conductive polymer having a low thermal conduction rate, such as a polyimide material, or other suitable type material. The insert is positioned in the insert aperture of a metal clamp insert carrier and extends therethrough to provide a peripheral clamping surface against the leadframe about a die adhered to a leadframe paddle. The insert and elastic member are held in place during clamping by a retainer, typically formed of a removable metal plate.
A variety of clamp inserts may be formed for differing semiconductor device/leadframe configurations. Each clamp insert may be used for wirebonding different semiconductor devices, thus minimizing the number of required inserts in a wirebonding operation. Thus, while the clamp inserts may be considered as product specific, each insert may typically accommodate a variety of package types or sizes. Moreover, the clamp inserts are easily and quickly exchanged i.e. installed and removed, for wirebonding different packages with the same insert carrier.
Using a generic insert carrier applicable to a particular leadframe width, inserts of various product-specific dimensions may be formed for wirebonding a wide variety of semiconductor packages including DIP, ZIP, TUF, SOJ, TSOP, PLCC, SOIC, PQFP, TQFP and IDF.
The thin member comprises a layer of a compressible elastic material which acts as a dampener and a self-leveling or self-adjusting device. Upward forces exerted by the leadframe against the insert by non-uniform leadframe width, etc. are absorbed by the layer of elastic material, ensuring a generally uniform clamping force over the entire clamping surface.
In addition, heat losses from the leadframe and die are much reduced by the clamp insert which has a low heat conductivity.
Furthermore, the electrically non-conductive nature of the clamp insert avoids damage to the die, leadframe and wires due to accidental electrical discharge, e.g. short circuits.